Arc lamp including electrodes having integral means for securing the electrodes against shock dislodgement

ABSTRACT

An arc lamp construction wherein a pair of metallic electrodes defining an arc gap are mounted within a quartz envelope and are secured within said envelope by shrinking the same into engagement with a foot portion of each electrode. The foot portions of the electrodes are tapered and include certain surface irregularities large enough to allow the quartz envelope, when shrunk thereabout, to be deformed into or around the irregularities to provide an interlocking mechanical mating therebetween to prevent longitudinal displacement of the electrodes within the envelope.

United States Patent Raymond E. Paquette Saratoga, Calif.

Apr. 4, 1969 Jan. 4, l 972 Republic National Bank of Dallas; Irving Trust Company; Union Bank Inventor Appl. No. Filed Patented Assignees ARC LAMP INCLUDING ELECTRODES HAVING INTEGRAL MEANS FOR SECURING TIIE ELECTRODES AGAINST SHOCK DISLODGEMENT 6 Claims, 8 Drawing Figs.

US. Cl 313/283, 313/217, 313/220 H01] 1/88 Field of Search 174/50, 61

I Will/12% References Cited UNITED STATES PATENTS 2,699,847 1/1955 Nelson et al 287/189365 Primary Examiner-Raymond F. Hossfeld Att0meyl-larvey G. Lowhurst ARC LAMP INCLUDING ELECTRODES HAVING INTEGRAL MEANS FOR SECURING THE ELECTRODES AGAINST SHOCK DISLODGEMEN'I BACKGROUND OF THE INVENTION The present invention relates generally to are lamps and, more particularly, to a novel means of securing the electrodes thereof within the housing envelope.

Arc lamps are used to provide a very intense source of light in many fields of application. In some of these applications, such as when the lamp is used as an optical tracking element in airborne vehicles and missiles, the operational environment is quite hostile to the lamp structure. In such applications, the lamp structure may be subjected to severe shock and vibration sufiicient to cause the electrodes to become dislodged from their seat within the envelope and thus cause inoperability or even destruction of the lamp.

In prior art are lamp structures, the electrodes have been secured within the envelope by merely shrinking the envelope around the electrode so as to form a relatively weak frictional engagement between the outer surface of the electrode and the inner surface of the envelope. Prior art lamps constructed in this manner are illustrated in the US. Pat. to Keller et al. No. 3,256,659 and Paquette et al. 3,274,427. This method of securing the electrodes within the housing, however, has been found inadequate for use in certain high shock applications. Apparatus having means for improving the frictional engagement between electrode and envelope are disclosed in the Paquette US. Pat. applications Ser. No. 618,438 filed Feb. 24, 1967 and Ser. No. 677,565 filed Oct. 24, 1967 now U.S. Pat. No. 3,518,480 both of which are assigned to the assignee of the present invention.

The principal problem encountered in securing the electrodes is a result of the difierence in the expansion coefficients of the materials. For example, if the envelope is shrunk down too tightly on the main body of the electrode, the differential expansion between the electrode and envelope under the extreme temperatures to which the electrode is subjected in operation may cause the envelope to be shattered by the electrode as it heats up. This problem can be circumvented by reducing the cross-sectional area of a distal portion of the electrode so that the relative cross-sectional area of the envelope with respect to the electrode is substantially larger than around the body portion of the electrode.

In so doing, the envelope can be shrunk down more tightly upon that portion of the electrode and is able to withstand the forces created by the differential expansion. However, the envelope still does not tightly engage the electrode when cold, again because the electrode will shrink more in cooling than the envelope. And, where the foot portion is tapered there may be a tendency to wedgedly displace the electrode in the direction of the arc gap as the device heats up in use. Means must thus be provided to prevent this type of unintentional dislodgement.

OBJECTS OF THE INVENTION It is therefore a principal object of the present invention to provide a novel electrode structure for short-arc lamps which will enable the electrodes to be firmly secured within the envelopehousing by a simple shrink fitting of the envelope.

Another object of the present invention is to provide a novel short-arc lamp apparatus which can withstand severe vibra tion and shock conditions without allowing the electrodes to become dislodged from their mounts within the envelope.

Still another object of the present invention is to provide a novel method of attaching an arc lamp electrode toa quartz envelope.

Still another object of the present invention is toprovide a novel are lamp electrode structure which can be fixedly secured by a mechanical interlocking with the quartz envelope by simply shrink fitting the envelope around a predetermined portion of the electrode.

SUMMARY OF THE PRESENT INVENTION In accordance with the present invention, a novel short-arc lamp electrode structure is provided having a foot portion which is tapered so as to have a cross-sectional area substantially less than the main body of the electrode. indentations, apertures, ridges or combinations thereof are formed in or on the tapered portion so as to allow the quartz envelope to flow into or around these features during the shrink fitting operation to provide an interlocking engagement between the electrode and the housing which can withstand severe shock and vibration.

The advantages of the novel electrode configuration will be apparent to those skilled in the art after a reading of the following disclosure of preferred embodiments which are illustrated in the several figures of the drawing.

IN THE DRAWING FIG. 1 is a partially broken away elevation of an arc lamp having electrodes in accordance with the present invention.

FIG. 2 is a partial section taken along the lines 2-2 of FIG. 1.

FIG. 3 illustrates an alternative embodiment of an electrode structure in accordance with the present invention.

FIG. 4 is a section taken along the lines 4-4 of FIG. 3.

FIG. 5 illustrates still another alternate embodiment of an electrode structure in accordance with the present invention.

FIG. 6 is a cross section of the alternate embodiment shown in FIG. 5 taken along the lines 6-6.

FIG. 7 illustrates still another alternate embodiment of an electrode structure in accordance with the present invention.

FIG. 8 is a section taken along the lines 8-8 of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2 of the drawing, there is shown a short-arc bridge-wire lamp 10 constructed in accordance with the present invention. The lamp 10 includes a lamp envelope 12 which is typically made of quartz having a thermal coefficient of linear expansion of about 49x10" per degree centigrade. Mounted within the lamp envelope 112 are a pair of electrodes 14 and 16 which respectively form the anode and cathode of the lamp 10. The electrodes 14 and 16 are typically made of a refractory metal such as swaged tungsten having a thermal coefficient of linear expansion of about 42X 10- per degree centigrade.

The electrodes 14 and 16 respectively include main body portions 18 and 20, foot portions 22 and 24 and are end portions 26 and 28. The electrodes 14 and I6 may be identical in size as illustrated in the drawing so that either may be selected as the cathode thereby making the lamp unpolarized. In the alternative, one of the electrodes may be constructed of a larger diameter than the other to form the hotter anode as is sometimes the practice in arc lamps. However, the fact that the lamp may be made nonpolarized is an advantage in that savings are made in manufacturing cost and the user can connect the voltage supply to the lamp without regard to polarity. If the lamp 10 is of the self-starting type, a bridge-wire 30 is typically connected across the electrode gap.

The foot portions 22 and 24 of the electrodes 14 and 16 respectively are tapered to provide a means for supporting the electrodes within the envelope and also to provide a means to which a flat metal interconnect ribbon 32 can be connected to either electrode. The electrodes 14 and 16 are primarily supported in cantilever fashion within the envelope 12 by shrinking the portions of the envelope 12 coextensive with the electrode feet 22 and 24 about the respective electrode foot.

In order to insure that the electrodes are fixedly secured within the envelope 12, the foot portions of the respective electrodes 14 and I6 are provided with indentations 34, the larger dimensions of which are wide enough to allow the softened quartz to flow thereinto. To insure that the softened quartz can flow into the indentations 34, the edge width thereof should be at least 0.062 inch and the depth should be at least 0.015 inch.

The conductive ribbons 32 are connected to the feet 22 and 24 by spot welds and are typically made of molybdenum having a width of about one-eighth inch and a thickness of about 0.7 mils. The other end of the ribbons 32 are similarly connected to terminal lugs '36 to which the external leads 38 may be attached. Alternate methods of attaching the ribbons 32 to the feet of the electrodes are disclosed in the aforementioned U.S. Pat. application Ser. No. 677,565 and in U.S. Pat. No. 3,532,922 both of which are assigned to the assignee of the present invention.

Before describing a number of possible modifications of the electrode-envelope interlock features in accordance with the present invention, it may be helpful to discuss the general support requirements which are met by the indentations 34. As a consequence of the cantilever support of the electrodes within the lamp envelope, the basic survival capability of the arc lamp to severe shock and vibration is low. Shrinking of the envelope upon the main body portion of the electrode when the same is below operating temperature is not practically possible in order to secure the electrode in place because the electrode has a much greater thermal coefiicient of linear expansion than the envelope and would, therefore, crack the envelope when expanding during arc operation.

Shrinking the envelope directly upon the body portion of the electrode when the same is at operating temperature is likewise not the answer because as the lamp cools the electrode shrinks away from the envelope and is thus not securely supported against lateral and longitudinal displacement when not in operation. Accordingly, what is required is a mechanical interlock between the electrodes and envelope which is not materially affected by the differential expansion when the temperature of the apparatus is changed. Another requirement, of course, is that the mechanical interlock itself does not become self-disintegrating due to expansion or contraction of the envelope and electrode as the temperature thereof is increased or decreased.

In accordance with the invention, such an interlocking means is provided and is illustrated in the drawing. The envelope intrusions 35 extend into the indentations 34 far enough so that even when the lamp is cold (as illustrated) the intrusions extend beyond the expansion gap separating the electrodes from the envelope and mesh with the electrode indentations 34. The shear strength of the intrusions 35 is more than adequate to sustain the shear load imposed thereon by the electrode as the lamp is subjected to severe acceleration.

In FIGS. 3 and 4 an alternative electrode structure is illustrated wherein the foot portion 40 of the electrode 42 has been extended somewhat and two pairs of indentations 44 have been provided in the sides thereof. As in the previously described embodiment, as the envelope is shrunk over the foot portion 40, the softened quartz of the envelope 12 will be caused to flow into the apertures 44 and provide the desired mechanical interlock.

In FIGS. and 6, still another modification of the present invention is illustrated which provides the same function as do the previously described embodiments but wherein the foot 46 carries the male portion of the mechanical interconnect arid the envelope forms the female portion. In this embodiment, one or more projections or ridges 48 are provided on either or both the top and bottom sides of the foot 46 so that the softened quartz, upon being shrunk around the foot portion 46, produces the desired mechanical interlocking feature.

In FIGS. 7 and 8, still another modification of the interlocking feature of the present invention is illustrated which is a combination of the previously described interlocking mechanisms. In this embodiment, a pair of indentations 50 are provided on one portion of the foot 52 so that the softened quartz can flow thereinto, and ridges 54 are provided on another portion of the foot 52 forming projections which extend into the quartz. This embodiment, of course, provides a combination of the advantages obtained in the other embodiment.

The shrinking of the envelope 12 around the electrodes to form the desired interlocking junctions is typically accomplished by an additional step in the manufacture of the lamp after the envelope has been initially shrunk around the electrode feet and sealed over the ribbons. More particularly, the arc lamp which is under negative pressure for well-known manufacturing reasons, is heated uniformly at a temperature corresponding to its operating temperature which is between 1,400 and 1,500 C. This causes the electrodes and the envelope to expand in accordance with the respective thermal coefficients of linear expansion and assure their sizes at operating conditions. Thereafter, the portions of the envelope l2 coextensive with the electrode feet are heated to about l,650 C., which is close to the melting point of the quartz. The softened quartz is thus caused to flow into the respective foot indentations or around the foot ridges.

After a suitable period of time, the arc lamp is allowed to cool and since the thermal coefiicient of linear expansion of the quartz is approximately 10 times less than that of the tungsten electrodes, the electrodes will shrink away from the quartz envelope so as to provide a separation therebetween. For an electrode having a diameter of about one-eighth of an inch, this separation has been found to be about 0.001 inch around the main body portion. It is substantially less, however, at the distal end of the tapered foot because of the reduction in cross section of the metal. But even so, were it not for the indentations and/or projections provided in the foot portions of the electrodes, the electrodes would still be more or less loosely fitted within the envelope 12 and would be restrained from axial movement therein only by the ribbons 32.

It is to be noted at this point that whenever the arc lamp is at its operating temperature, a hard shrink condition exists between the envelope and the electrodes. This condition is not dangerous to the integrity of the envelope since the envelope was shrunk onto the electrode at this temperature and the forces due to expansion of the electrode are not in excess of those which cause the envelope to shrink about the electrode in the first place during manufacture. Although this hard shrink condition is sufficient to provide stability against lateral movement of the electrode within the envelope, the frictional engagement between the electrode and envelope is not sufficient to assure against dislodgement in the longitudinal direction.

However, because of the mechanical interlocks provided by the indentations or projections on the foot portions of the electrodes, such longitudinal restriction is provided which can sustain severe shocks and vibrational conditions. Accordingly, during the operating conditions, there is both firm lateral and longitudinal support of the electrodes and good thermal contact between the electrode and the envelope due to the hard shrink conditions.

As previously pointed out, when the arc lamp is cold, the indentations and/or projections on the foot portions of the electrode still provide the desired interconnect since the mating penetration exceeds the differential expansion and will continue to provide the intended mechanical interlock even though the electrode has shrunk away from a tight frictional engagement with the envelope.

Although specific examples of interlock configurations have been illustrated in the drawing, the invention is not intended to be limited to these particular configurations. There are merely configurations that have been found convenient to form in the extremely hard tungsten material from which the electrodes are made. The scope of the invention is intended to extend to any type of aperture, indentation, projection, ridge, serration or the like in the foot portion of the electrodes which will provide the desired interlock between the electrodes and the envelope. Thus, an arc lamp has been described wherein the possibility of damage from vibration and shock is substantially reduced and alignment of the electrodes is facilitated and maintained thereby improving the overall dependability of the lamp.

While the above detailed description has shown, described What is claimed is:

1. An arc lamp comprising:

a pair of axially aligned spaced-apart electrodes defining an arc gap therebetween, each of said electrodes including, a generally cylindrical body portion of a predetermined diameter, and a tapered foot portion forming one end of said body portion, said foot portion terminating in a generally rectangular cross section having a width substantially equal to the diameter of said body portion and a thickness substantially less than the diameter of said body portion, said foot portion including surface irregularities large enough to form a mechanical interlock with molten quartz shrunk thereabout;

a generally cylindrical quartz envelope means hermetically enclosing said electrodes, said envelope means being heat shrunk about the foot portion of each of said electrodes thereby supporting said electrodes in cantilever fashion within said envelope; and

conductor means weldedly attached to each of said foot portions and extending outside of said envelope means to provide electrical connection to said electrodes.

2. An arc lamp as recited in claim ll wherein said surface ir regularities include indentations of dimensions large enough to allow molten quartz to flow thereinto.

3. An arc lamp as recited in claim 2 wherein said indentations have an edge width of at least 0.062 inch and a depth of at least 0.0 l 5 inch.

4 An arc lamp as recited in claim 2 wherein said surface irregularities further include projections from the surface of said foot portion, said projections extending away from said surface a distance greater than any differential shrinkage separation between said foot portion and said envelope means at room temperature.

5. An arc lamp as recited in claim 1 wherein said surface ir regularities include projections from the surface of said foot portion, said projections extending away from said surface a distance greater than any differential shrinkage separation between said foot portion and said envelope means at room temperature.

6. An arc lamp as recited in claim 5 wherein said surface irregularities further include indentations of dimensions suffi cient to allow molten quartz to flow thereinto. 

1. An arc lamp comprising: a pair of axially aligned spaced-apart electrodes defining an arc gap therebetween, each of said electrodes including, a generally cylindrical body portion of a predetermined diameter, and a tapered foot portion forming one end of said body portion, said foot portion terminating in a generally rectangular cross section having a width substantially equal to the diameter of said body portion and a thickness substantially less than the diameter of said body portion, said foot portion including surface irregularities large enough to form a mechanical interlock with molten quartz shrunk thereabout; a generally cylindrical quartz envelope means hermetically enclosing said electrodes, said envelope means being heat shrunk about the foot portion of each of said electrodes thereby supporting said electrodes in cantilever fashion within said envelope; and conductor means weldedly attached to each of said foot portions and extending outside of said envelope means to provide electrical connection to said electrodes.
 2. An arc lamp as recited in claim 1 wherein said surface irregularities include indentations of dimensions large enough to allow molten quartz to flow thereinto.
 3. An arc lamp as recited in claim 2 wherein said indentations have an edge width of at least 0.062 inch and a depth of at least 0.015 inch.
 4. An arc lamp as recited in claim 2 wherein said surface irregularities further include projections from the surface of said foot portion, said projections extending away from said surface a distance greater than any differential shrinkage separation between said foot portion and said envelope means at room temperature.
 5. An arc lamp as recited in claim 1 wherein said surface irregularities include projections from the surface of said foot portion, said projections extending away from said surface a distance greater than any differential shrinkage separation between said foot portion and sAid envelope means at room temperature.
 6. An arc lamp as recited in claim 5 wherein said surface irregularities further include indentations of dimensions sufficient to allow molten quartz to flow thereinto. 